Europe
Canada thistle (Cirsium arvense)
Larvae are barrel-shaped, white, have dark brown anal plates, and can be up to 5 mm long at maturity. Adults can be up to 8 mm long and have dark bodies and dark bands on their white wings that form a ‘W’ pattern. Males have a rounded abdomen and females have a long, pointed ovipositor for laying eggs.
Overwintering larvae pupate inside galls in early spring as plants start bolting. Adults emerge in late spring and early summer when new Canada thistle stems are bolting and forming buds. Eggs are deposited on plant shoots in axillary buds throughout summer. Hatching larvae burrow into stems and cause galls to form. There are often multiple larvae in one gall. Larvae overwinter in the third (final) instar. There is one generation per year.
Larval feeding causes gall formation. Galls act as metabolic sinks, diverting
Randall, C.B., J.E., Andreas, J. Milan, and K. Gladem. 2024. Introduced Thistle Biocontrol Agents: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2024-8-INTRODUCED THISTLES-A. https://bugwoodcloud.org/resource/files/29169.pdf